vector write/read

[Florian Weimer]

* Alan Bateman:

> Off-hand, I don't know if this is an area that has been standarized
> (the closest that comes to mind is lio_listio in the real-time
> extensions when used in wait mode).

On recent Linux, it should be possible to fake the read case using
posix_fadvise and pread (the interesting property is reordering among
multiple reads).  For the write case, pwrite can be used directly
because there's no implied ordering constraint.

(It's unlikely that the savings due to reduced syscall overhead are
actually relevant on Linux.)